Distortion correction for transmission lines



Aug. 21, 1928.

O H. NYQUIST DISTORTION CORRECTION FOR TRANSMISSION LINES Filed Sept.11, 1926 INVENTOR. qaz'sl,

ATTORNEYS.

Patented Aug. 21, 1928.

UNITED STATES PATENT OFFICE.

HARRY NYQUIS'I, OI MILLBURN, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE-AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

DIS'IORTION CORRECTION FOR TRANSMISSION LINES.

Application tiled September 11, 1926. Serial No. 184,980.

This invention relates to the correction bf distortion in transmissionlines, and more particularly to a method and means for correcting forthe difference in time of transmission and in the attenuation of thecomponents of a complex wave, such as an electrical signal Wave, over atransmission line.

In transmission lines such as are used for signalling purposes, thevelocity of transmission and the attenuation depend on the frequency,and as a result the various components making up the initial wave formare changed in magnitude and phase, resulting in a substantialdistortion which becomes greater as the length of the line is increased.

Various attempts have been made to correct for this distortion, thisbeing commonly accomplished by the introduction of electrical networksof such characteristics as to introduce additional delay and attenuationin those portions of the frequency spectrum in which these are low, andby suitable design of the electrical networks it is possible to give atransmission line a practically uniform characteristic throughout thatportion of the frequency spectrum of interest. Such correction networksare illustrated in 'my application, Serial No. 751,146, filed November20, 1924.

For long lines, the cost of a sufliciently large number of sections tointroduce the necessary delay or correction may become rohibitive andthe object of this invention.

1s to provide a method of and means for doing this which is moreeconomical than the use of electric circuits. I accomplish this byobtaining part of the correction by a mechanical delay device such as aphonograph, a telegraphone or other e uiva ent means, and then obtainingthe remainder of the necessary or desirable correction by a limitednumber of sections of electric networks. A further object of myinvention then is to obtain the major portion of correction for such atransmission line' by economical and convenient mechanical means and toconfine the use of electrical sections to the final correction.

' In a long transmission line there is quite commonly excessive delay incertain portions of the frequency spectrum which introduces specialdifficulties to the problem. In'this invention I overcome suchdiificuilty by the method of splitting a wide transmission band offrequencies into several narrow'bands by means of filters, equalizingseparatelyeach band by suitable phase and attenuation equalizers, andfinally recombining the separate bands in a single circuit. In the caseof very long circuits, where the delay required in the equalizingapparatus is very great, it may be possible to select, by means offilters, one or more parts of a band where a greatand approximatelyconstantdelay may be utilized. This part of the total delay may beobtained by various substitute electromechanical devices referred toabove and which may be mentioned here more specifically. For example,one might use any of the following devices: (a) A phonograph, whenreproducing needle follows recording needle after a given time andrecording .and reproducing attachments are connected so as to operatefrom or work into an electric circuit, respectively; (b) a telegraphoneof thekind now wellknown in the art and referred to more specifically inFigure 3 of the accompan ing drawing, to which further reference will bemade below; (a) the speaking tube described in my gppization, Serial No.127,934, filed August The invention will be better understood byreference to the following specification and the accompanying'drawing,1nwhich Figure 1 shows the manner of splitting up the frequency spectrumfor a given transmission line and means for correcting each narrow band;Fig. 2 shows a delay characteristic curve of a long transmission lineand the application of the arrangement of Fig. 1 towards its correction;Fig. 3 gives one form of mechanical delay device for the purposementioned heretofore; Figs. 4, 4 and 4 represent typical forms oflattice-type networks useful for the final correction; and Fig. 5 showscurves giving the delay characteristics of such networks.

illustrate how the reduction in the number ofelectrical networks maybe'accomplished, let us assume that it is desired to use the method ofthis invention in correcting distor- 100 special purpose it is desiredto transmit a 105 band from 300 to 2300 cycles. It is then seen fromFig. 2 that a constant delay of .1 of a second added from 300 to 1600cycles raises that portion of the delay curve to d, ,e, and

the area covered by the lifting of this portion 110 I In order toexplain the invention and to of the curve gives the area A which isequal to 130 units, the unit here being an arbitrary one. A constantdelay of .06 seconds added from 1600 to 2000 cycles gives the curve f,.gand in being so lifted the curve covers the area B equal to 24 units. Ifnow, in Fig. 1 the transmission line is divided into three portionsextending, respectively, from 300 to 1600 c cles, 1600 to 2000 cycles,and 2000 to 2300 cyc es, by means of accurate sharp cutoff filters, wemay then introduce electro-mechanical devices A and B, as shown in Fig.1, in the channels transmitting the two lower frequency bands so as toyield a resultant delay given by the curve at, e, f, g, h, c.

Having thus corrected for the major portion of the delay, it will now befeasible to correct each of the three channels of Fig. 1 still furtherwith electrical networks so as to take care of the delay represented bythe areas C, D and E of Fig. 2 yielding such resultant delay as may bedesirable, which, in general, would be a straight line, as indicated byl, m of Fig. 2. This correction may be accomplished by any suitableelectrical networks such as are indicated in Fig. 1 by C, D and E, andit will be convenient at the same time to make these electrical networksof such form as to equalize for variable attenuation throughout thefrequency spectrum in which we may be interested.

Certain forms of networks for phase correction are shown in Figs. 4, 4and 4, which may be made electrically equivalent by giving proper valuesto the reactances, one form of network being more convenient in certaincases than in others. These electrical networks are explained in detailin my copending applications, Serial No. 751,146, filed November 20,1924, and Serial No. 90,656 of February 26, 1926, which applicationsrelate specifically to the correction for transmission lines, thecorrection being obtained by electrical networks. In Fig. 5, there isshown a group of curves giving the delay characteristics of suchnetworks in terms of the natural frequency of the members of thenetworks of Figs. 4, 4 and 4. The delay characteristic for any one ofthese networks, as ex lained in my co-pending applications, may econtrolled, and this is indicated in Fig. 5 by the curves havingdifferent values for a parameter 6, which is determined by the relativemagnitudes of the inductances and the capacities in the network inquestion. By the choice of'one or more of these networks of givencharacteristic, it is possible to introuce almost any desired delay inany portion of the frequency spectrum, and by the choice of a pluralityof these networks, with the same or different values for the parameterb, it is possible to introduce almost any desired delay at any frequencyor band of frequencies. The area under each of the curves of Fig. 5 hasthe property of being equal to one of the arbitrary units heretoforementioned, of which there were 130 in the area A. By using at suchsections, the total delay throughout the frequency spectrum as a wholewill be equal to 7!. units. It is apparent, therefore, that in order toobtain the total correction corresponding to the curve a, Z), c of Fig.2, a large number of sections would be required, but having obtained themajor portion of the correction by mechanical means, a relatively fewnumber of sections of the electrical networks will be necessary to takecare of the balanceof the correction.

The saving in the number of sections of electrical networks due to thissubstitution can be computed for any case. In the particular one inhand, let us again suppose that the delay of the total transmitted band,300 to 2300 cycles, is first equalized by means of N sections ofelectrical network without splitting the bands by means of filters. Inthis case, we will have the relation N,=A+B+C+D+E+n, where n, is thearea under the total delay curve of N, sections which occurs outside ofthe range of 300 to 2300 cycles. If, now, the areas A and B are takencare of electromechanically, we need only the following number ofsections of electrical networks to take care of the areas C, D and E,respectively,

'N,=C+n., 2 N D+7ll 3 N.,=E+n. 4

where m, m and n, are the areas under the delay frequency curves lyingoutside of the transmitted bands of the respective channels. Thus, thesaving S in total sections of electrical networks is S=N,N.N.;N 5=A-l-B-i-n,'n n -n. 6 =154+'n,n n n, 7

It is economical to make 71,, m n, and n, as small as possible byroperly selecting the values of b and f for t e various sections ofelectrical networks, this being ossible as is seen by the curves of Fig.5. us, the total saving in electrical sections of distortion correctionnetworks obtained by using the electro-mechanical devices amounts toabout 154 sections.

As indicated above, the mechanical delay may be obtained by any one of avariety of mechanical devices, of which only one need here beillustrated. Fig. 3 shows such a mechanical delay device, thisconsisting of a telegraphone which comprises a magnetizable wire orribbon 10, moving continuously in any suitable manner, such as overwheels 11 and 12. Adjacent to this moving wire, is a recording magnet14, which is connected to the incomin channel of one branch of Fig. 1. Asecon magnetic winding 15 then picks up and reproduces the signal,transmitting it to the output terminals of this same channel. By movingthe reproducer 15 to or from the recorder 14, any desired delay may beobtained. Instead of moving the magnets, obviously the same result couldbe accomplished by changing the velocity of the moving Wire or ribbon.An electro-magnet 16 adjacent the wire should be provided for wipingofithe record and placing the wire in condition for operation on againreaching the recorder 14. This electro-magnet may be excited by a directcurrent source or a high frequency alternating current source, as ispreferred.

Having introduced the mechanical delays A and B in two of the channels,and having made the final correction by means of the electricalnetworks, the three channels of Fig. 1 may then be brought together, andthe signal allowed to proceed into a single output channel.

While I have described this invention in connection with specificapparatus, it is to be understood that the invention should not be solimited, but that it is capable of numerous modifications withoutdeparting from the spirit of my invention.

What is claimed is:

1. In a transmission system, the method of correcting for delayirregularities which consists in making the large portion of the delaycorrection mechanically and the final portions electrically.

2. In a transmission system, the method of correcting for delay andattenuation irregularities which consists in correcting for the mainportion of the delay mechanically and in correcting for the attenuationand the final portions of the delay electrically.

3. In a transmission system, the method of correcting for delayirregularities which consists in dividing the transmission channel intoa plurality of parallel branches, dividing the frequency spectrum amongthese branches according to extent of delay, making the main delaycorrection in each branch mechanically and the final delay correction ineach branch electrically.

4. In a transmission line to be corrected for delay irregularities, asection of said line divided into a plurality of parallel branches, aWave filter in each sectionadapted to pass a definite frequency band,mechanical delay devices in each branch adapted to correct for most ofthe delay, and electrical networks in each branch adapted to correct forthe remainder of the delay.

5. In a transmission line to be corrected for delay irregularities, asection of said line divided into a plurality of parallel branches,means in each branch adapted to pass a definite frequency band only,mechanical delay devices in a branch carrying'a band with large delayand electrical delay devices in various branches to correct for smallerdelays.

In testimony whereof, I have signed my name to this specification this10th day of September, 1926.

' HARRY NYQUIST.

